Biochemists enjoy the freedom to purify and study enzymes in isolation, saturate an enzyme with its substrate, trap/remove the products, as also provide optimal pH, ionic strength, etc. On the other hand, cell extracts are by their very nature “dirty enzymes”; intact cells and organisms are “dirtier” still. The cell by design is greatly constrained to provide a consensus medium to simultaneously support hundreds of diverse enzyme-catalyzed reactions. Only some of these enzymes may be operating under optimal conditions at any time. The context for an enzyme to function in vivo is very different from the well-defined conditions deliberately set up for its study in vitro. And classical biochemistry is founded on several assumptions valid in dilute aqueous solutions. These assumptions are often extended without question to the cellular milieu. But the cell interior is far away from being an ideal solution. The key features that differentiate the situation in vivo from that in vitro are cataloged below.